This invention concerns a method and means for measuring high frequency analog signals, such as vibrations from a building, bridges, a car or other body to which a sensor may be attached using a fiber optic sensing device. The applications for such sensors are many, for example, vibration sensors mounted on automobiles could be used to provide a signal to a servo control which could adjust air shock absorbers to provide an extremely smooth ride. Vibration sensors could be mounted on buildings which are under construction in order to detect large motions and provide alarms which would warn construction workers of approaching collapse preventing tragedies prevalent at such sites. Monitoring bridges such as the one which collapsed in Connecticut a few years ago could also present tragedies.
Another application for vibration sensors is intrusion detection for security systems. Conventional devices often fail due to false alarms caused by detection of low frequency signals such as vibrations of a truck on a nearby street. The optical configuration described here would be configured to discriminate against such low frequency signals. Sensing other phenomena such as thermal and ultrasonic variations when measured using this sensing technique also have applications for security systems as well as other systems.
Conventional sensors for measuring vibratory motion, dynamic thermal fluctuations or other physical phenomena of interest are typically designed to sense low frequency variations which are characterized by signals of relatively large amplitude. As the frequency associated with the phenomena to be measured, e.g., vibrations, increases, the signal amplitude decreases except in the region of resonances.
Sensors currently available in the market have flat frequency responses such that their ability to detect signals at higher frequencies decreases. That is, the dynamic range of sensors with flat frequency responses is limited by the large amplitudes generated by virtually every natural phenomena at low frequencies. Using vibrations as an example, a typical sensor which exhibits a flat response with respect to frequency and which has limited sensitivity for detecting high frequency vibrations is the PCB Model 303A02 accelerometer. When used with the amplifier designed for this accelerometer, it is limited to 1.times.10.sup.-5 gs at 8 kHz.
The response curves for currently available accelerometers may be plotted. Assuming, for example, that the amplitude of the signal to be measured, e.g., vibrations, decreases linearly with an increase in frequency, the transfer function of sensors currently available to measure such signals are flat with respect to frequency such that the relative output of sensors decreases linearly as frequency increases. For example, assume that the minimum detectable signal of a typical sensor is 1 at 1 Hertz with a relative output signal of 1000 at that frequency. The transfer function for that sensor dictates a signal to noise ratio of only 5 at a frequency of 1 kHz.
The sensor of the present invention overcomes these difficulties of the prior art sensors. It employs an optical configuration which provides a transfer function which increases linearly with frequency, thus providing greater sensitivity at higher frequencies. As used herein, the term high frequency is a relative term. It can in an extreme example be only a few Hertz. Typically, the current frequency ranges within the meaning of this term are 100 Hz to 100 kHz.
The configuration of the invention described herein provides optimum response maximizing both high frequency sensitivity and dynamic range of the sensor. The transfer function for the device of the invention increases linearly with frequency. Thus, in the example given above on the assumption of boundaries of 1 Hertz and 1 kHz, the signal to noise ratio of the sensor of the current invention is 1000 at both boundaries.
This optical sensor configuration is ideal for measuring high frequency signals and can be utilized for sensing virtually any parameter which can be measured with an optical fiber interferometer, e.g., acoustic, magnetic, electrical fields, thermal fluctuations, vibrations, strains and so forth. Fiber optic accelerometers are known but they too have flat frequency responses and, consequently, poor minimum detection thresholds at high frequencies. They are also sensitive to interference from low frequency environmental factors such as temperature variations.